The present invention relates to the field of laser welding and in particular to a laser welding head and also to a welding method using the head.
Laser welding is a technique that enables a plurality of parts made of meltable material, and in particular of metal material, to be assembled together by delivering energy by means of a laser beam. The laser beam is focused on a focal point between two adjacent parts and serves to heat the material for welding locally to above its melting point, thereby enabling the two parts to be welded together in a join plane. Progressively advancing the welding point perpendicularly to the focusing axis of the laser beam then enables the adjacent parts to be joined together along a line of welding following this direction of advance.
In order to avoid oxidation of the material in the line of welding, the person skilled in the art knows that an inert protective gas, such as argon for example, can be injected onto the focal point through an annular nozzle situated around the focusing axis.
Nevertheless, a drawback of that technique lies in progressive deterioration of the optical element used for directing and focusing the laser, and in particular of the focusing lens located at the end of the light path. Since this lens is exposed to an environment that is aggressive, with hot gas, dust, and droplets of molten metal, it can become degraded quickly. Unfortunately, the cost of such optical elements is very high, and replacing them frequently can significantly penalize the use of a laser welding device.
Placing a protective pane in front of the focusing lens enables this drawback to be solved in very imperfect manner only. Even if such a pane is less expensive than a focusing lens, frequently replacing it as a result of its own degradation also constitutes a significant cost, not only because of the cost of replacing it, but also because of the need to stop the laser welding device in order to replace the pane.
Proposals have thus been made to protect the focusing lens by a transverse flow of air flowing past the front of the lens in order to keep hot gas, dust, and droplets away that might otherwise degrade the surface of the focusing lens. Nevertheless, the effectiveness of such a “crossjet” technique can be limited. In particular, if it is combined with injecting a protective gas around the focusing axis, aerodynamic interactions between the transverse flow of air and the jet of protective gas can destabilize the jet of protective gas, thereby exposing the molten material to undesired oxidation.